The Impact of Production Techniques on Pore Size Distribution in High-Strength Foam Concrete

Abstract

This study examined the impact of various foam concrete production techniques on pore size distribution and its water absorption properties. Techniques such as the use of a cavitation disintegrator and a turbulent mixer were employed to produce foam concrete. Six foam concrete compositions, with dry densities ranging from 820 to 1480 kg/m³ and compressive strength up to 47 MPa, were prepared. A novel method for digital image correlation was applied to analyse the pore size distribution within the foam concrete specimens. The manufactured foam concrete specimens’ porosity and water absorption indices were determined. The experimental results, including compression strength and water absorption, indicated that the production technique significantly affects the pore size distribution in foam concrete, impacting its mechanical and durability properties. Compressive strength was assessed at curing intervals of 7, 28, and 180 days. Cavitation technology was found to promote the formation of a finer porous structure in foam concrete, resulting in enhanced strength properties.

1. Introduction

Foam concrete (FC) is a lightweight cementitious material with a cellular structure produced by incorporating the air voids into the cement-based matrix. It can be designed to have any density within the range of 200–1900 kg/m³. FC is characterised by high flowability, low cement and aggregate content, relatively low strength, excellent thermal insulation, high fire resistance, and good sound insulation properties [1,2,3,4,5,6,7]. It should be noted that FC is a universal material suitable for application as a heat-insulating material, as well as for self-supporting and load-bearing elements [8].

The properties of the FC are influenced by the mixture components, their proportions, and the mixing method. Key factors include the type and content of cement, the water-to-cement or water-to-binder ratio, the grading distribution, the filler-to-cement ratio, and the type of foaming agent (FA), all of which play a significant role. The strength of FC is closely associated with its density, with lower densities typically leading to reduced strength. For example, decreasing the water-to-cement ratio from 0.5 to 0.38 has been found to result in an 18.1% increase in compressive strength [9].

The strength of FC is generally significantly lower than that of normal-weight concrete due to its lower density and higher porosity [10]. However, when higher densities are achieved—up to approximately 1900 kg/m³—foam concrete can attain strengths exceeding 50 MPa [11]. This performance enhancement at higher densities is primarily attributed to the reduction in pore size and volume, along with a denser matrix structure. Such high-strength foam concrete finds applications in structural elements where weight reduction and adequate load-bearing capacity are both critical requirements.

FC can be produced using various techniques, with two primary methods being widely utilised: (1) the pre-foaming method and (2) the mixed foaming method, also referred to as the inline method or high-speed mixing method.

In the pre-foaming method (1), the cement-based matrix and preformed foam are prepared separately. First, all ingredients, except for the foam, i.e., constituents of the cement-based matrix, are mixed until a homogeneous matrix with a uniform consistency is achieved. Once the cement-based matrix is ready, the required preformed foam is added. In general, a mixing time of 2–3 min is sufficient to integrate the foam into the matrix; however, the mixing duration can vary depending on the output of the foam generator and the desired concrete density [12]. A low mixing speed is recommended to avoid bubble breakage, especially when dealing with a paste with high viscosity.

In contrast, the mixed foaming method (2) involves producing foam in concrete by mixing the foaming agent directly with the matrix constituents. Usually, the foaming agent is added along with water and the base mix ingredients during the mixing process [3,13]. After forming the foam, the other constituents are introduced into the same mixer and combined until the desired density is achieved. This method is standardised, widely used, and simple to follow [3]. A key advantage of the mixed foaming method is that it simplifies the production process by eliminating the need for separate foam production.

In both production techniques—(1) the pre-foaming method and (2) the mixed foaming method—the foam must remain stable to withstand the pressure from the mortar until the initial set. This stability is crucial for forming a strong concrete structure around the air-filled voids, ensuring the integrity of the foam concrete [7]. The methodology for the assessment of the foam stability was presented earlier in [14]. Notably, the pre-foaming method is more expensive than the mixed foaming method, but it offers better foaming efficiency without compromising the quality of the air bubbles in the mixture, i.e., destroying air bubbles [15]. Furthermore, the pre-foaming method is preferred for producing low-density FCs. At the same time, the amount of air bubbles in the mixture is easier to control by adding a specific volume of preformed foam. In contrast, in the mixed foaming method, the air content depends on various factors that are difficult to control; for instance, the quantity of foaming agent needs to be determined for a particular mixer.

Pore size distribution is one of the critical parameters that significantly influences the physical, mechanical, and durability properties of FC. In particular, the mixture consistency and the production technique highly affect the pore size distribution. Larger pores (macro-pores) weaken the material by serving as stress concentration points that can trigger cracking under load. In contrast, a more significant presence of smaller pores (micro-pores) promotes a more uniform stress distribution, enhancing the material’s strength. A previous study [16] found that a finer pore distribution and more concentrated and uniform porosity play a key role in the development of high-strength foam concrete (FC). This emphasises the importance of controlling pore size distribution during FC production to achieve an optimal balance between density and strength. An effective FC mix design should focus on minimising large pores while maximising the presence of well-distributed micro-pores, enhancing the structural performance of the material.

Amran et al. [7] indicated that an ideal pore size distribution, with a higher proportion of small, evenly distributed pores, can significantly improve the compressive strength of FC. However, an overly fine pore structure may lead to shrinkage during curing, resulting in micro-cracks that could weaken the material’s structural integrity [17]. Ramamurthy et al. [3] also highlighted the critical role of pore structure in influencing the strength of FC. The authors concluded that FC exhibiting a narrower air-void size distribution demonstrates greater strength. Specifically, a finer distribution of smaller air voids can lead to a denser microstructure and improved strength, even if the overall porosity remains constant. Generally, there is a fundamental inverse relationship between porosity and the FC strength [15]. This relationship indicates that as porosity increases, the strength of the material typically decreases.

Pore size distribution also affects the water absorption characteristics and, consequently, the durability of FC [18,19]. Larger pores allow for more water ingress, which can lead to freeze–thaw damage, efflorescence, and other durability issues. On the other hand, FC with a higher proportion of micro-pores shows lower water permeability, making it more resistant to environmental degradation. Moreover, the connectivity of pores is important. Well-connected pores in FC can negatively affect its durability by allowing capillary water to penetrate more easily into the material. This water can lead to several issues, such as freeze–thaw damage and corrosion of reinforcement. Higher water absorption can lead to greater susceptibility to chemical attacks, such as sulphate or acid reactions, which degrade the concrete. Therefore, controlling the distribution and connectivity of pore sizes is key to enhancing the long-term durability of FC, especially in environments exposed to moisture.

The production technique is crucial in determining the size, shape, and distribution of pores—whether macro- or micro-pores—in FC. As such, this study focused on examining how different production techniques influence the pore size distribution and, in turn, the water absorption characteristics of FC. The primary aim was to develop a highly efficient, high-strength FC by employing innovative production methods. Additionally, the study aims to identify correlations between the qualitative aspects of pore distribution and the physical and mechanical properties of the hardened material. Understanding these relationships is vital for optimising both the performance and durability of foam concrete in various applications.

2. Materials and Methods

2.1. Overview of the Mixers Used

The FC mixers used in this study are presented in Figure 1. The specifications of the mixers used are outlined below.

(a) Cavitation Disintegrator (CD): Originally, the CD was developed for producing stable fuel mixtures and water–fuel emulsions and improving heavy fuel oil by dispersing asphalt–resin components to enhance the efficiency of added substances. It can also be applied to prepare and activate other emulsion and dispersion systems [20]. Mironovs et al. [21] patented a novel hydrodynamic dispersion method for fine particles, achieved through mechanical activation driven by cavitation, which occurs during the high-speed rotation of toothed disks (up to 7000 rpm). The use of CD for FC production is an innovative approach. Since CD is not widely known in the construction industry, its technical features are worth noting. In CD, a set of toothed disks and an impeller, mounted on a motorised shaft, form a rotor with a frustoconical shape. The fixed body consists of a conical lid and rectangular grooves matching the rotor’s toothed disks (see Figure 1a). The body contains an inlet and outlet branch pipe. The electric motor is shielded from liquid by a ferrule installed in an intermediate disk, which is fastened to the motor flange. A plug is installed in the upper part of the body to discharge air. The cement-based matrix, along with the foaming agent, is first placed in the supply reservoir. During operation, the mixture circulates repeatedly from the supply reservoir through a tube into the disintegrator. The CD used in this study had an output capacity of 10 m³/h, a power rating of 5.5 kW, and was equipped with a frequency converter to adjust the working shaft’s speed from 50 rpm to 7000 rpm.

(b) Turbulence Mixer (TM): The TM used in this study enables the production of FC using both the mixed-foaming and pre-foaming methods. The TM can also be used to produce cement-based slurries and pump them using air pressure. The mixer consists of a conical mixing tank, electric motor, bearing, pressure-compensated coupling, and a vertical shaft, as depicted in Figure 1b. The mixer allows mixing under pressure up to 0.7 bar, enabling the production of low-density FC and the pumping of the mixture through excess pressure. It is equipped with a frequency converter to control the shaft speed from 20 rpm to 1000 rpm. As studied earlier in [6], a key advantage of this mixer, especially when used in conjunction with 3D printing concrete (3DPC), is its ability to pump FC directly, eliminating the need for a separate screw pump. According to the TM mixer’s technical data, the FC produced under applied air pressure can be pumped up to a distance of 100 m.

2.2. Raw Materials and Mixtures Under Investigation

Type II Portland composite cement CEM II/A-M (S-LL) 52.5 R (OPTERRA Zement GmbH, Werk Karsdorf, Germany) was used in the production of various mixtures. Hard coal fly ash Steament H4 (STEAG Power Minerals GmbH, Dinslaken, Germany) was chosen as a secondary cementitious material. The chemical composition of cementitious materials is presented in

Table 1. Chemical composition of cement and fly ash.

Material	Chemical Composition (%)
	Density (kg/m3)	Residue	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	Ka2O	Na2O	Loss on Ign.	CO2	CL
CEM II/A-M(S-LL) 52.5 R	3.120	0.74	20.63	5.35	2.82	60.94	2.14	3.52	1.05	0.22	3.47	2.87	0.07
Fly ash H4	2.220					3.6		0.6		2.9	1.8		<0.01

. A polycarboxylate ether (PCE)-based superplasticiser (SP) (MasterGlenium SKY 593, BASF Construction Solutions GmbH, Trostberg, Germany) was used in the cement-based matrix to adjust the workability at reduced water contents. The SP is characterised by a density of 1050 kg/m³ and a water content of 77% by mass. Two different foaming agents (FAs) were used for the production of the FC: a tenside-based foaming agent (Centripor SK155, MC-Bauchemie GmbH & Co. KG, Bottrop, Germany) and a protein-based foaming agent (Oxal PLB6, MC-Bauchemie GmbH & Co. KG, Bottrop, Germany).

The particle size distribution of the cementitious materials used in the current study is summarised in Figure 2. According to the results, fly ash has slightly larger particle sizes than Portland cement.

The designed FC mix compositions are presented in

Table 2. Compositions of the foam concretes.

Constituents		Designed Composition of Mixture (Per m³)
		M-1	M-2	M-3	M-4	M-5	M-6
Cement	(kg)	405	405	405	405	405	405
Fly ash H4	(kg)	192	192	192	192	192	192
Tap water	(kg)	189	189	243	189	189	192
SP SKY 593	(%) *	0.74	1.03	0.15	0.74	0.74	0.88
FA SK-155	(%) *	1.2	1.2	1.2	-	0.7	0.7
FA Oxal PLB6	(%) *	-	-	-	1.2	-	-
w/b	-	0.39	0.39	0.5	0.39	0.39	0.4
Used Mixer		CD	CD	CD	CD	TM	TM

* In the mass percentage of the cement.

. The ratio of components is given in mass proportions. Compositions from M1 to M4 were prepared in a cavitation disintegrator, and compositions M5 and M6 were prepared in a turbulent mixer.

The differences in the physical and mechanical properties of compositions M5 and M6 can be explained by the high sensitivity of the mixed foaming method. Even minor changes in the amount of water and plasticising additives may lead to the disruption of the foaming process and changes in the pore structure. As shown in Figure 4f in Section 2.4, the M6 composition has more coarse-sized cells that could have been formed by merging previously formed smaller cells. This could be caused by increased consumption of the plasticiser.

2.3. Experimental Procedure

Dry constituents in proportions according to the mix design in Table 2 were initially mixed with the addition of water and SP for 2 min using a brick trowel. Following the premixing, the foaming agent was added and mixed for an additional 15 s. Finally, the mixture was then transferred to either the CD or TM mixer, depending on the technique used. Considering the technical characteristics of these mixers, the mixing protocols for FC production differed; details are provided in

Table 3. Foam concrete mixing procedure.

Cavitation Disintegrator (CD)	Turbulence Mixer (TM)
0 min–2.0 min: 2100 rpm 2.0 min–4.0 min: 2400 rpm 4.0 min–6.0 min: 3600 rpm 6.0 min: Conveying of the FC with settled ca. 2100 rpm	0 min–2.0 min: 1500 rpm 2.0 min–4.5 min: 3000 rpm 4.5 min–6.0 min: 4800 rpm 6.0 min: Setting of 1 bar air pressure for conveying of the FC

. After mixing, the FC was cast into moulds. In the case of the CD mixer, the FC circulated numerous times from the supply reservoir through the tube to the disintegrator. At the end of the mixing procedure, the CD’s pumping ability allowed the FC to be conveyed directly to the prepared moulds rather than returning to the supply reservoir. For the TM mixer, a pressure of 1.0 bar was settled to transport the FC to the moulds, which were then covered with polyethylene foil to prevent water evaporation. After 24 h, the specimens were unmoulded, wrapped into a polyethylene foil, and stored under constant temperature and humidity conditions (+20 ± 2 °C, 50%) until testing. The water absorption of the designed FC compositions was measured following EN 772-11 [22]. The dry density of the FC was determined using prism specimens with dimensions of approx. 150 × 40 × 40 mm³, which were dried in a drying chamber at a temperature of +105 ± 5 °C until a constant mass was achieved. The compressive strength was determined following EN 12390-3 [23].

2.4. Specimen Preparation and Image Processing

There is currently no standardised method for determining the pore size distribution in FC. In addition, the variety of pore classes and surface properties of different FCs complicates establishing a universal measurement method suitable for all types of FC. This study used an automated, software-supported image analysis method to measure pore size distribution. Figure 3 shows the optical digital microscope VHX 600 (Keyence Deutschland GmbH, Neu-Isenburg, Germany), equipped with a high-resolution image analysis tool used for digital photography and image processing. The determination of the pore size distribution and the measurement of porosity were performed on samples with dimensions of 150 × 100 × 40 mm according to EN 480-11 [24]. These samples were extracted from casted specimens with dimensions 150 × 150 × 500 mm. Three samples were cut from each specimen and taken from the middle third of the produced specimens. The analysed surface area corresponded to the cross-section of the prism specimens, ranging between 148 and 150 mm². To determine the relevant pore size parameters, the following preparation steps were necessary:

• Polishing of the surface with sandpaper of different grain sizes in uprate from the grain size of 300 µm to 1000 µm;
• Dyeing of the polished surface with a black felt-tip pen;
• Filling of the pores with a contrasting colour powder (white BaSO₄).

The black-and-white contrast enabled the simple binarisation of the images. In this process, black areas correspond to the non-porous surfaces of the sample, whereas the white colour depicts the pores. In the binary image, the number of pixels of each colour is counted and referenced to a unit area, e.g., the number of pores per square centimetre. Figure 4 shows a typical binary image of the FCs, with the pictures representing the analysed mix compositions in the current study.

3. Results and Discussion

3.1. Porosity Measurements

Figure 5 shows the measured porosity of six different FC compositions. Compositions M-3-CD and M-5-CD exhibit highest porosities at 53.9% and 48.9%, respectively. Conversely, the M-4-CD composition shows the lowest porosity at 14.3%, which can be explained by the ineffectiveness of the protein-based foaming agent when used with the CD mixer. It is anticipated that a protein-based foaming agent would also be inefficient with the mixed-foaming technique using the TM mixer. This assumption was confirmed in subsequent tests with the TM mixer during the compilation of this manuscript. When comparing the performance of the two different mixers, it is evident that the TM mixer can achieve higher foaming of the cement-based matrix even with a lower dosage of foaming agent than the CD mixer. This can be clearly seen by comparing the M-1-CD composition, which has a porosity of 30.4%, with the M-5-TM composition, which has a porosity of 48.9%. The amount of foaming agent used in the M-1-CD composition was higher than that in the M-5-TM composition. These observations are further supported by the results of dry density presented in Figure 6.

The results of the measured porosity of the samples correlate with the measured dry density, as shown in Figure 6. As a result of the study, a direct relationship was found between the porosity obtained by digital image analysis methods and the dry-state density of the material. This correlation shows that digital image analysis can be considered an alternative method for quantification of material porosity. Since porosity and the pore structure influence the mechanical properties of the FC, the measured porosity can be interlinked with mechanical properties. However, to establish a reliable relationship between sample porosity and mechanical properties, a larger database on material properties is needed. In this study, the decision to focus on a single cross-section per sample was made based on the goal of providing a preliminary insight into the influence of production techniques on pore size distribution in high-strength foam concrete. Given the controlled conditions of the production process, the selected cross-section was deemed representative of the overall pore structure for each production technique. While additional cross-sectional data could provide further statistical depth, the primary aim was to compare the qualitative differences in pore formation. Expanding the study to include more cross-sections could be valuable in future research, but for the scope of this work, the approach used allowed for sufficient analysis of the observed trends.

3.2. Air-Void Size Distribution

The pore diameters relative to the number of pores are shown in

Table 4. Pore size distribution.

Max. Diameter in [mm]	M-1-CD	M-2-CD	M-3-CD	M-4-CD	M-5-TM	M-6-TM
0–0.1	4349	7919	6607	16,731	4282	7053
0.1–0.2	1114	1969	1207	2153	1289	939
0.2–0.3	344	505	179	191	258	279
0.3–0.4	129	229	62	43	109	99
0.4–0.5	72	109	35	6	74	56
0.5–0.6	35	62	20	6	39	36
0.6–0.7	36	34	13	1	43	30
0.7–0.8	12	16	10	1	22	12
0.8–0.9	12	11	8	0	14	8
0.9–1	9	9	10	0	28	22
>1	13	11	22	3	43	26
Total	6125	10,874	8173	19,135	6201	8560

. It is noteworthy that the number of counted pores is most variable within the 0–0.1 mm range, with this range showing the highest count compared to other pore size groups. The M-5-TM composition, with a porosity of 48.9% and a dry density of 946 kg/m³, exhibits a lower total number of pores. This means that the larger pores, with diameters exceeding approximately 0.4 mm, are primarily responsible for the high porosity of the sample; see Figure 6. In contrast, the M-3-CD composition, which has a porosity of 53.9%, shows a significantly higher total pore count than the M-5-TM composition. The sample of composition M-4-CD with the lowest porosity and the highest dry density has the highest total number of pores compared to all compositions. This observation leads to the conclusion that the total number of counted pores or the number of pores in certain groups does not necessarily provide a reliable basis for comparison of the porosity of the sample across different mix compositions.

Figure 7 presents the cumulative frequency distribution of the pores based on the occupied area. Analysing the relationship between porosity and pore size distribution, it can be concluded that specimen M-4-CD, which has the lowest porosity, contains the smallest number of pores with a size greater than 0.4 mm. However, these larger pores are responsible for the total porosity of the specimen.

Analysing the obtained results, it can be seen that the lowest values of cumulative area of frequency in the pore range of 0–0.5 mm are found for the M-4-CD composition, which also has the highest density and strength indicators. The highest values of the cumulative area frequency are observed for M-1-CD, M-2-CD, and M-3-CD compositions, which were prepared in a cavitation disintegrator. These compositions have high-frequency values in the range of 0–0.5 mm. The compositions that were prepared in a turbulent mixer (M-5-TM and M-6-TM) have a smoother pore distribution curve, with a lower content of small pores in the range of 0–0.2 mm.

3.3. Compressive Strength

Figure 8 shows the results of determining the compressive strength at different curing times of 7, 28, and 180 days. The obtained curves show a well-confirmed relationship between the strength of FC and its density.

The obtained results confirmed a fundamental inverse relationship between porosity and strength, as mentioned in [15]. Summarising the strength results, it should be noted that despite the fact that manufacturing technology has some effect on strength, the determining factors are the material’s density and porosity.

Figure 9 presents the relationship between the area of small air voids (0–0.1 mm) and compressive strength at 28 days.

It can be concluded that there is a direct relationship (correlation factor R² = 0.904) between the area of small pores and the 28-day strength of FC. The M-4-CD composition has the highest strength and the highest number of small pores. In contrast, the M-3-CD composition, which has a low value of small pores, has the lowest compressive strength of 9.4 MPa.

The results of the relationship between density, strength, and pore distribution are consistent with those of other researchers. For example, in [25], it is shown that high-strength FC has a fine-sized pore structure and a lower total porosity.

3.4. Water Absorption

Analysing the relationship between water absorption and porosity (see Figure 10), it can be concluded that, with an increase in porosity, there is an accelerated increase in water absorption, which can be explained by the more open nature of the pores. The M-4-CD composition with the lowest porosity is also characterised by the lowest water absorption rate (14.4%). The highest porosity and water absorption value is shown by composition M-3-CD, which also has the lowest density. Additionally, from the previous graphs, it can also be concluded that water absorption decreases with an increase in the area of small pores (0–0.1 mm).

4. Conclusions

This research study has indicated how different production techniques can significantly impact the material properties of foam concrete. The pore structure of foam concrete significantly influences its thermal insulation properties, compressive strength, and durability [3]. For example, controlling the pore size distribution, as shown in our study, can improve the material’s insulation properties, which is essential for energy-efficient building construction. Furthermore, finer and more uniformly distributed pores may increase the overall strength of the concrete, as noted by [3,10,25], who reported that pore size has a direct correlation with mechanical performance. Our study emphasises the importance of carefully selecting production techniques to optimise these properties for use in sustainable and high-performance construction materials.

In particular, this study highlights how varying methods of FC production, such as the cavitation disintegrator (CD) and turbulence mixer (TM), affect not only the porosity but also the overall durability, strength, and performance of the material. By comparing these techniques and obtained properties, the study provides insights into how production choices influence the FC’s vulnerability to environmental factors and its long-term structural integrity. The outcome of the study can be concluded as follows:

• Both the TM and CD mixers demonstrated superior performance in FC preparation using the mixed-foaming method. With the CD mixer, a wide range of densities from 820 to 1480 kg/m³ was achieved. However, the TM mixer allowed FC to be obtained with a lower dosage of foaming agent.
• Porosity measurements were correlated with dry density, suggesting that digital image analysis is a viable method for quantifying material properties. However, a larger dataset is needed to link porosity with mechanical properties reliably.
• The number of pores was most variable in the 0–0.1 mm range, and the larger pores over 0.4 mm significantly influenced the overall porosity.
• Water absorption increased with porosity, but it was also influenced by pore distribution and shape. Therefore, it was affected by both, not just the total porosity.
• A 28-day compressive strength ranging from 9.4 to 47.4 MPa was achieved. Moreover, the highest compressive strength was exhibited by the M-4-CD composition, characterised by the lowest porosity and the highest frequency of small pores in the range of 0–0.1 mm.
• Similarly, the finest porous composition, M-4-CD, had the lowest water absorption rate, which can be mainly explained by the closed nature of the pores.
• By ensuring the hardening process, the strength of the specimens increased with age. At 180 days, the compressive strength increased by 10–15% compared to the 28-day results, with the composition M-4-CD reaching 53 MPa at a density of 1480 kg/m³.

These findings highlight the importance of carefully selecting production techniques to optimise FC’s structural and durability properties. Future studies should expand the dataset to better establish the relationship between porosity and mechanical strength and refine pore characterisation techniques using advanced imaging methods. Also, future research could include a more extensive statistical analysis involving multiple cross-sections of each sample. This would provide a broader understanding of how pore size distribution varies across different parts of a foam concrete sample and could help validate the representativeness of a single cross-section. Additionally, the use of advanced imaging techniques such as micro-computed tomography (micro-CT) scanning could offer a more detailed three-dimensional view of the pore structure and allow for more precise quantification of pore size and distribution. Such approaches would improve the reliability and depth of analysis and provide a more comprehensive understanding of the factors influencing foam concrete’s structural properties.